ABSTRACT Most adults in the U.S. fail to meet national physical activity (PA) and dietary recommendations, and Latinos are less likely to meet these recommendations than non-Latino Whites. Physical inactivity, a poor diet, and obesity are major risk factors for cardiovascular disease, certain cancers, diabetes, and stroke and are important contributors to preventable morbidity and mortality in the U.S. Social environments are widely recognized to have an important impact on these behaviors, yet most behavior change programs give limited attention to the social contexts in which behaviors occur. The goal of the proposed research is to evaluate the efficacy of a 6-month, community-based family dyad intervention in promoting engagement in and maintenance of PA and healthy eating among Latino adults residing in three predominantly Latino communities in Houston, TX. Dyads will be randomly assigned to a community health worker (CHW)-led intervention, ?Tu Salud, Si Cuenta! ? Familiar? (TSSC-Family) or to a contact control condition focused on healthy homes. TSSC-Family will consist of six home visits delivered to dyad members over a six-month period to build behavioral skills and social support for sustained healthy behavior change among Latino family dyads. The healthy homes contact control will also consist of six monthly visits delivered to dyad members by a CHW. Study participants will include 552 Latino adults not currently meeting PA or nutrition recommendations recruited and enrolled as family member dyads (n=276 dyads). This approach expands on an empirically- based motivational behavior change intervention developed by a member of the investigative team in collaboration with community partners that has shown preliminary evidence of effectiveness for positive PA and dietary behavior change in Latino individuals. Participants will be assessed at baseline, 6 months, and 12 months after baseline to evaluate both intermediate and long-term effects. Primary outcomes include minutes per week of moderate and vigorous PA, and servings of fruits and vegetables, sugar-sweetened beverages, and whole grains. Weight loss is a secondary outcome. Self-efficacy, stage of change, social support, and social control will also be examined as potential mediators of changes in PA and nutrition. The proposed research is innovative in its use of a novel intervention that emphasizes dyadic social processes in addition to standard behavior change strategies. The intervention explicitly targets family-based social networks to foster social environments supportive of healthy behavior change using CHWs to facilitate home-based outreach and communication to this understudied population. This research is expected to yield critical insight regarding effective approaches for increasing PA and healthy eating among Latinos, the largest and among the fastest growing minority groups in the U.S.